Iron ore is an important natural resource and iron may be the world's most commonly used metal. Iron may be extracted from iron ore and used in a variety of commercial and industrial applications, including the manufacture of steel. Typically, iron extraction from iron ore results in certain byproducts that still include some remaining iron. These byproducts are generally considered waste, especially if the iron cannot be economically extracted from the slag.
Iron is generally extracted from iron ore rocks that contain enough metallic iron for economical extraction. The iron in iron ore is generally found in the form of magnetite, hematite, taconite, goethite, limonite, and siderite, for example. Iron ore is mainly made of iron ore oxides carrying different quantities of iron. For instance, based on the respective atomic numbers of iron (Fe)—55.84—and oxygen (O)—15.994—we see that a typical iron ore molecule of Fe2O3 carries close to 70% of iron by weight. One main use of iron ore having high iron content (e.g. greater than about 60%), is to produce “pig iron.” Pig iron, a main material used to make steel, is an intermediate product resulting from the reduction of the iron ore through the smelting of iron ore with a carbon fuel such as coke, charcoal, and anthracite. Pig iron is mainly made of iron with a high carbon content residue of the reduction process. Pig iron is commonly processed in and poured directly from a blast furnace for transfer to a steel mill. It is noted that, while iron ore may be a suitable feed for blast furnaces of integrated steel mills, the iron ore is not suitable for the minimills of the steel industry, which commonly rely on electric arc furnaces to produce steel. Instead, the minimills require to be fed with higher iron content material like pig iron and steel scrap. In steel processing, for example, pig iron from blast furnaces is used to produce steel, usually with an electric arc, induction, or oxygen furnace, by burning off excess carbon and adding certain metal alloys.
As an alternative to processing (reducing) iron ore in a blast furnace to produce pig iron, new technologies have been developed to process iron ore by direct reduction to produce iron nuggets or pellets suitable as a substitute for pig iron in minimill steel production. For example, new direct reduction processes have achieved the production of metallic iron nuggets having a metallic iron content greater than 90%, sometimes as high as 97%, using iron ore as feed. These iron nuggets are well suited for use in electric arc furnaces in place of pig iron.
The direct reduction techniques replaces the work of certain processing plants and sometimes eliminates the need for coke ovens. The process generates less emissions, less energy, and offers lower overall costs than traditional processes for the generation of pig iron. The direct reduction process is more energy efficient than the blast furnace because it operates at a lower temperature, and there are several other factors that make the direct reduction process economical. In certain direct reduction techniques, iron ore nuggets are produced in a rotary hearth furnace using a feed of iron ore (in the form of lumps, pellets, or fines) using a reducing gas produced from natural gas or coal. The reducing gas is a mixture majority of hydrogen and carbon monoxide which acts as a reducing agent.
Conventional byproduct processing techniques have relied upon magnets to further extract iron from processing byproducts, since iron is magnetic. However, for the byproducts of direct reduction techniques, magnetic separation has been found relatively ineffective. For example, certain non-ferrous elements in the byproducts may have been magnetized through the reduction process, making the magnetic separation of these byproducts less desirable as the resulting product will include these impurities. Also the possible significant quantity of iron in the byproducts of a direct reduction technique increases the likelihood that non-ferrous elements will get trapped between the iron particles as they attach to the magnetic surface, also reducing the iron purity of the output.
The byproducts of direct reduction, however, have proven to be difficult to process, especially using conventional magnetic techniques, although still containing valuable elements such as iron and anthracite, for example. Examples of direct reduction byproducts include “iron fines” mixed with dust. Iron fines include particles having a size of 15 mm or less consisting of iron slag and anthracite, for example. The iron fine byproduct may consist of about 60% or less metallic iron. The byproducts of direct reduction may also include iron slag consisting of about 10% or less magnetic iron, without (or with less) dust or anthracite, for example, and “revert,” which is a byproduct consisting of a combination of coke, iron slag, and anthracite.
What is needed is a process to recover iron from an iron byproduct, to reduce the amount of waste from mining and reduction operations, and to provide a valuable resource for the economy. Further, it is preferable that the recovery process is a dry process, as iron is prone to oxidize (rust) in the presence of water.